Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by a relatively high metals content. This occurs because substantially all of the metals present in the original crude remain in the residual fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper sometimes present.
The high metals content of the residual fractions generally preclude their effective use as chargestocks for subsequent catalytic processing such as catalytic cracking and hydrocracking, because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree.-1100.degree. F. temperature and a pressure of 1-10 atmospheres. The economic value of the coke byproduct is determined by its quality, particularly its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts per million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high-valued metallurgical, electrical, and mechanical applications.
Presently, catalytic cracking is generally accomplished by utilizing hydrocarbon chargestocks lighter than residual fractions which usually have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, and the like, the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree.-1500.degree. F., a pressure of about 1-5 atmospheres, and a space velocity of about 1-1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5-25, or even 2.5-50, may be used to blend with or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80 percent of the metals and preferably at least 90 percent needs to be removed to produce fractions (having a metals factor of about 2.5-50) suitable for cracking chargestocks.
The automatic and environmental factors relating to upgrading of petroleum residual oils and other heavy hydrocarbon feedstocks have encouraged efforts to provide improved processing technology, as exemplified by the disclosures of various U.S. patents which include U.S. Pat. Nos. 3,696,027; 3,730,879; 3,775,303; 3,876,530; 3,882,049; 3,897,329; 3,905,893; 3,901,792; 3,964,995; 3,985,643; 4,016,067, and the like.
Another important trend with respect to the conservation and efficient conversion of energy resources is the development of improved technology to increase the yield of liquid fuels per barrel of gas oil feedstock processed in refinery operations.
In a conventional catalytic cracking system, a portion of the gas oil charge is refractory and not easily converted to lighter products. The refractory hydrocarbon fraction is recovered by distillation and is accumulated on a continuous basis. If the refractory material is recycled, it contributes to coke formation on the catalyst and yields little additional lighter product. The refractory material is also high in metals content which contributes to deactivation of cracking catalysts.
Another increasingly significant undertaking is the development of alternatives to petroleum as sources for fuels and chemical intermediates, e.g., coal in particular because of its relative abundance and availability.
Since most current energy utilization technology requires liquid energy media, it has become an important research and development objective to provide innovative means to convert coal into liquid sources of potential energy.
It was recognized by early workers that coal can be liquefied by controlled heating in the substantial absence of oxygen. The conversion products are a liquid and a char. Because of the new compelling economic factors, the technology of coal liquefaction and gasification has been expanding at an accelerated pace. Pioneer developments in the field are represented by Lurgi and Fischer-Tropsch technology.
A broad variety of organic solvents have been proposed for solubilizing coal. Most of the solvent media have disadvantages of high cost, poor solvation capacity for coal constituents, high viscosity, and the like. Coal tar, recycle coal oil, petroleum refinery byproduct streams, and propane-deasphalted petroleum tar, are among the coal solvation solvents disclosed in the prior art. Recent advances in coal liquefaction are described in U.S. Pat. Nos. 1,904,586; 1,955,041; 1,996,009; 2,091,354; 2,174,184; 2,714,086; 3,375,188; 3,379,638; 3,607,718; 3,640,816; 3,642,608; 3,705,092; 3,849,287; 3,870,621; inter alia.
There remains a need for improved technology for the conversion of coal into liquid carbonaceous products to complement and to enhance conventional petroleum-derived commodities.
Accordingly, it is an object of this invention to provide an improved method for upgrading heavy hydrocarbon oils for use as demetallized and desulfurized feedstocks for petroleum refinery cracking operations.
It is another object of this invention to provide a means for converting accumulated refractory petroleum residua from refinery operations into liquid fuel range distillates.
It is a further object of this invention to provide an improved method for converting coal and other solid carbonaceous materials into liquid hydrocarbon derivatives.
Other objects and advantages of the present invention shall become apparent from the accompanying description and illustrated data.